1961: Robert M. White flew the X-15A-2, with the XLR99 57,000-pound thrust engine, on its first Mach 4 flight.
In February 1961, Maj. White unofficially set a new air speed record when he flew the X-15 at a speed of 2,275 mph, following the installation of a 57,000 lbf (250 kN) thrust XLR-99 engine. White was the first human to fly an aircraft at Mach 4 (7 March) and later Mach 5 over the next eight months.